Leaves’ ability to photosynthesize – the process by which they make energy from carbon dioxide, sunlight and water – begins to fail when their temperature reaches around 46.7 degrees Celsius (116 Fahrenheit).
While this may seem high, leaves can get much hotter than the air temperature, according to the report published Wednesday in Nature by a group of scientists from countries including the US, Australia and Brazil.
The scientists used temperature data beamed down from thermal satellite sensors on the International Space Station, 400 kilometers (nearly 250 miles) above the Earth. They combined this with on-the-ground observations from leaf-warming experiments, in which scientists climbed into the canopy to painstakingly add sensors to leaves.
Rather than looking at average temperatures, the scientists were looking at extremes, said Christopher Doughty, associate professor in ecoinformatics at Northern Arizona University and a report author. They found that average forest canopy temperatures peaked at 34 degrees Celsius (93 Fahrenheit) but some exceeded 40 degrees Celsius (104 Fahrenheit).
Currently, 0.01% of leaves are passing the critical temperature threshold beyond which their ability to photosynthesize breaks down, the report found, potentially killing the leaf and the tree.
This percentage, while small, is poised to increase as the world warms, the report said, posing a threat to the world’s tropical forests – which cover roughly 12% of the planet and hold more than half of the world’s species. They also provide a vital role in sucking up and storing carbon and helping to regulate the global climate.
“There are all sorts of potential feedbacks once you start losing bits of forests, even leaves on individual trees,” Doughty said on a call with reporters.
Tropical forests can withstand around 4 degrees Celsius (7.2 Fahrenheit) of additional global warming before they reach a tipping point in terms of their ability to photosynthesize, according to the report.
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If warming exceeds this level, the amount of leaves surpassing critical temperature thresholds could rise to 1.4%, potentially causing large-scale leaf loss and the death of the whole tree, the report found.
This level of warming is not expected under current climate policies, which are estimated to bring 2.7 degrees Celsius of warming above pre-industrial levels. But it is within the range of the most pessimistic climate scenarios if the world continues to burn fossil fuels.
While these figures may seem small, the risk is significant given how critical tropical trees are for life, the climate system and the planet.
“Almost all life – including humans – is dependent on photosynthesis for food either directly or indirectly,” said Kevin Collins, senior lecturer in environment and systems at the Open University, who was not involved in the research.
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The findings indicate that global warming is putting this vital process at risk, he told the Science Media Centre. But, he added, there are more immediate concerns for tropical forests, including deforestation, wildfires and droughts.
Christopher Still, a forest ecosystem professor at Oregon State University, also not involved in the report, said that the research provided some novel insights. While there’s been a big focus on the impact of drought on tree loss, he told CNN, “this paper says it’s not just drought – we need to really also worry about the temperatures that leaves are reaching.”
While he acknowledged the numbers in the report are small, he added, “I would focus a little bit less on the percentages, and more on the concepts of leaves pushing up against these really high temperature conditions and how often that happens, how long it lasts and what it really means.”
Others were more cautious. Chloe Brimicombe, a climate scientist at the University of Graz in Austria, said that given how few leaves are reaching the critical temperature threshold, and how high warming would need to get before a tipping point, “this suggests in theory tropical forests are quite resilient to climate change.”
She told the Science Media Centre, “this is a simple model and trees and forest dynamics are much more complex than this.”
The report authors said despite uncertainties, the research gives important insights around how tropical forests will respond to climate change.
“It’s a little bit of a canary in the coal mine that we’re starting to see,” Joshua Fisher, a climate scientist at Chapman University and a report author, said on a call with reporters. “And you want to be able to detect something happening before it’s widespread.”
